Sieradz is a city on the Warta river in central Poland with 40,891 inhabitants. It is the seat of the Sieradz County, situated in the Łódź …Sieradz is a city on the Warta river in central Poland with 40,891 inhabitants. It is the seat of the Sieradz County, situated in the Łódź Voivodeship. Sieradz is a capital of the historical Sieradz Land.